Flash tube construction



April 4, 1967 MELA 3,312,853

FLASH TUBE CONSTRUCTION Filed Dec. 1, 1964 INVENTOR. RICHARD L. MELA BYw wh ATTORN EYS United States Patent Office 3,312,853 FLASH TUBECONSTRUCTION Richard L. Mela, Arlington, Mass., assignor to DynatechCorporation, Cambridge, Mass. Filed Dec. 1, 1964, Ser. No. 415,143 4Claims. (Cl. 313-218) This invention relates to a novel flash tube. Moreparticularly it relates to a flash tube of the type used to generatelight by passing an electric current through a gaseous medium disposedin the region between a pair of electrodes. The electrodes are made ofpyrolytic graphite, which exhibits characteristics providing an improvedlife span for the tube.

Generally speaking a flash tube of the type to which the invention isdirected comprises a quartz tube with electrodes disposed at oppositeends thereof. The tube is filled with a rarefied gas. The gas isinitially ionized by any one of several systems, for example, radiationby a high frequency signal, and the resulting low electrical resistancebetween the electrodes permits the passage of a large current throughthe tube to generate a flash of light. The life of a flash tube isadversely affected by a number of factors. Among these is electrodedeterioration. The electrodes are subject to intensive electron and ionbombardment as a result of the passage of current through the tube.This, together with the heat generated in the gaseous plasma,substantially elevates the electrode temperatures. In fact, levels ofseveral thousand degrees Kelvin may be reached in the tungstenelectrodes conventionally used. At such temperatures further electrodebombardment causes displacement of significant amounts of material fromthe surfaces of the electrodes. This sputtering action results in thedeposition of the material on the inner surfaces of the quartz tube. Inturn the coating of these surfaces reduces the transmission of lightfrom the tube.

Moreover, it appears that the larger droplets of electrode materialdeposited on the quartz surfaces concentrate thermal or mechanicalstresses, thereby hastening the ultimate cracking of the tube.

The deterioration of the electrode surfaces resulting from removal ofmaterial therefrom may also have deleterious effects on the operation ofthe flash tube, depending on the particular electric field configurationand current distribution desired in the vicinity of the electrodes. Incases where these factors are largely dependent on electrode surfaceshapes, operation can be materially degraded by surface deterioration.

Accordingly, it is a principal object of the invention to provide aflash tube characterized by an improved life span.

A more specific object of the invention is to alleviate thelife-shortening factors associated with electrode deterioration in priorflash tubes.

Yet another object of the invention is to provide a flash tubecharacterized by reduced rejection of material from electrodes,particularly in the form of relatively large droplets.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the features of construction,combination of elements, and arrangement of parts which will beexemplifiedin the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawing, which is a longitudinalsection of a flash tube embodying the invention.

The invention makes use of pyrolytic graphite as elec- 3,312,853Patented Apr. 4, 1967 trode material. This material has two propertieswhich are materially better than those of the conventionally usedtungsten and thereby significantly increase the life of the flash tube.These properties are the higher melting (or sublimation) point and thehigh thermal conductivity along the crystalline axis exhibited bypyrolytic graphite. The first of these properties provides a reductionin the amount of material lost from the electrodes during operation ofthe flash tube.

The higher thermal conductivity, an order of magnitude greater than thatof tungsten in the 3,000 K. range, results in better transfer of heatfrom the surface of the electrode to the exterior of the flash tubewhere it is readily dissipated. Thus, there is a smaller thermalgradient along the electrode and a correspondingly lower temperature onthe surface thereof at which the electron and ion bombardment takesplace. The lower temperature combines with the higher melting point toprovide a marked reduction in the amount of material removed from theelectrode surface. This in turn has been found to increase significantlythe operating life of the flash tube.

As shown in the drawing, a flash tube embodying the invention includes aquartz tube 10 having end caps 14 and 16 suitably sealed thereto.Extending through the cap 14 is an electrode assembly generallyindicated at 18. The assembly 18 includes a metallic bar 20 on the innerend of which is an electrode 22. If desired, the bar 20 may besurrounded by a glass liner 24, aflixed to both the bar 20 and the cap14 so as to help prevent leakage between the exterior and interior ofthe tube 10 along the assembly 18.

The electrode 22 is made of pyrolyticgraphite, with the axis thereofparallel to the axis of the tube 10, i.e. extending between the tip 22aof the electrode and the bar 20. The electrode 22 is fastened to the bar20 by any suitable technique providing a low thermal impedance at thejuncture of the two parts, for example by silver soldering. The bar 20is of a high conductivity material such as copper which readily conductsheat from the electrode 22 to the exterior of the tube 10. It ispreferably oxygen-free copper in order to minimize contamination of theinterior of the flash tube. Alternatively, the bar 20 may be an integralextension of the pyrolytic graphite electrode 22, i.e. the entirestructure 20-22 may be of this material.

The opposite end of the flash tube 10 is fitted with a second electrodeassembly generally indicated at 26. The assembly 26 includes a copperbar 28 extending through the cap 16 and joined to the end of a pyrolyticgraphite electrode 30. A glass liner 32 surrounds the bar 28. The

various parts of the electrode assembly 26 are similar to, and serve thesame function as, the corresponding parts of the assembly 18.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficient attained and,since certain changes may be made in the aboveconstruction withoutdeparting from the scope of the invention, it is intended that allmatter contained in the above description or shown in the accompanyingdrawing shall be interpreted as illustrative and not in a limitingsense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

I claim:

1. In a flash device of the type comprising an enclosure defining achamber containing a gas to be ionized to provide a flash, electrodesspaced apart within said chamber for passage of an electric currentthrough said gas between tips of said electrodes and means forconducting ing substantially between the tip of the electrode and theelectrical conducting means connected thereto.

2. A flash device comprising (A) enclosure means providing a chambercontaining a gas to be ionized,

(B) first and second electrode assemblies, each of said electrodeassemblies including (1) an electrode having a tip thereon,

(2) electrical conducting means extending into said chamber from theexterior of said enclosure means and connecting with an end of saidelectrode remote from said tip, 7

(C) said first and second electrodes being located and oriented withrespect to each other to provide for passage of an electric currentbetween said tips thereof by way of said gas,

(D) at least one of said electrodes being of mon-ocrystalline pyrolyticgraphite with the axis of maximum thermal conductivity of said graphiteextending substantially in the direction from the tip of said electrodeto the electrical conducting means connecting therewith.

3. In a flash device of the type comprising an enclosure defining achamber containing a gas to be ionized to provide a flash, electrodesspaced apart within said chamber for passage of an electric currentthrough said gas between tips of said electrodes and means having arelatively high thermal conductivity for conducting heat from saidelectrodes to the exterior of said chamber, the improvement in which atleast one of said electrodes is of pyrolytic graphite having an axis ofmaximum thermal conductivity, said axis extending from the tip of saidelectrode to the thermal conducting means conducting heat therefrom.

4. The combination defined in claim 3 in which both electrodes are ofpyrolytic graphite having an axis of maximum thermal conductivity, saidaxis of each electrode extending from the tip of said electrode to thethermal conducting means conducting heat therefrom.

References Cited by the Examiner UNITED STATES PATENTS 2,154,317 4/1939Suits 313-184 2,459,579 1/1949 Noel 3l3184 2,703,374 3/1955 Fruengel313-484 2,716,713 8/1955 Noel 313--218 3,107,180 10/1963 Diefendorf117226 JAMES W. LAWRENCE, Primary Examiner.

C. R. CAMPBELL, Assistant Examiner.

2. A FLASH DEVICE COMPRISING (A) ENCLOSURE MEANS PROVIDING A CHAMBERCONTAINING A GAS TO BE IONIZED, (B) FIRST AND SECOND ELECTRODEASSEMBLIES, EACH OF SAID ELECTRODE ASSEMBLIES INCLUDING (1) AN ELECTRODEHAVING A TIP THEREON, (2) ELECTRICAL CONDUCTING MEANS EXTENDING INTOSAID CHAMBER FROM THE EXTERIOR OF SAID ENCLOSURE MEANS AND CONNECTINGWITH AN END OF SAID ELECTRODE REMOTE FROM SAID TIP, (C) SAID FIRST ANDSECOND ELECTRODES BEING LOCATED AND ORIENTED WITH RESPECT TO EACH OTHERTO PROVIDE FOR PASSAGE OF AN ELECTRIC CURRENT BETWEEN SAID TIPS THEREOFBY WAY OF GAS, (D) AT LEAST ONE OF SAID ELECTRODES BEING OFMONOCRYSTALLINE PYROLYTIC GRAPHITE WITH THE AXIS OF MAXIMUM THERMALCONDUCTIVITY OF SAID GRAPHITE EXTENDING SUBSTANTIALLY IN THE DIRECTIONFROM THE TIP OF SAID ELECTRODE TO THE ELECTRICAL CONDUCTING MEANSCONNECTING THEREWITH.